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<i class='fas fa-caret-right ms-1'></i></div><div id=sidebar-toc class="hidden md:block overflow-y-auto mx-6 md:mx-0 pe-6 pt-2 md:max-h-doc-sidebar bg-primary-bg md:bg-transparent"><div class="flex flex-wrap ms-4 -me-2 p-2 bg-secondary-bg md:bg-primary-bg rounded"><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/>汇编语言</a></div><ul class=ps-6><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/18/>11.9 ~ 11.11</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/17/>11.7 ~ 11.9</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/16/>11.1 ~ 11.6</a></div></li><li class=py-2><div><a class="text-eureka hover:text-eureka" href=https://b.hui.ke/docs/assembly-language/15/>10.8 ~ 10.12</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/14/>10.1 ~ 10.7</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/13/>9.7 ~ 9.9</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/12/>9.1 ~ 9.6</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/11/>8.5 ~ 8.9</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/10/>8.1</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/9/>7.1 ~ 7.10</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/8/>6.1 ~ 6.3</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/7/>5.1 ~ 5.4</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/6/>4.1 ~ 4.9</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/5/>3.6 ~ 3.10</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/4/>3.1 ~ 3.5</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/3/>2.8 ~ 2.12</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/2/>1.11 ~ 2.6</a></div></li><li class=py-2><div><a class=hover:text-eureka href=https://b.hui.ke/docs/assembly-language/1/>1.1 ~ 1.10</a></div></li></ul></div></div></div><div class="w-full md:w-3/4 lg:w-4/5 pb-8 pt-2 md:pt-8"><div class=flex><div class="w-full lg:w-3/4 px-6"><article class=prose><h1 class=mb-4>10.8 ~ 10.12</h1><div class="text-tertiary-text not-prose mt-2 flex flex-row flex-wrap items-center"><div class="me-6 my-2"><i class="fas fa-calendar me-1"></i>
<span>2020-11-08</span></div><div class="me-6 my-2"><i class="fa-solid fa-pen-to-square me-1"></i>
<span>2020-11-08</span></div><div class="me-6 my-2"><i class="fas fa-clock me-1"></i>
<span>4 min read</span></div><div class="me-6 my-2"><i class="fas fa-folder me-1"></i>
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<a href=https://b.hui.ke/series/x86asm-language/ class=hover:text-eureka>x86asm Language</a></div><div class="me-6 my-2"><i class="fa-solid fa-eye me-1"></i>
<span id=busuanzi_value_page_pv><i class="fa fa-spinner fa-spin"></i></span> Hits</div></div><p>  《汇编语言（第3版）》10.8 ~ 10.12、《零基础入门学习汇编语言》P49 ~ 51</p><h3 id=108-mul-指令>10.8 mul 指令</h3><p>  因下面要用到，我们介绍一下 mul 指令，mul 是乘法指令，使用 mul 做乘法的时候</p><ol><li>相乘的两个数：要么<strong>都</strong>是 8 位，要么<strong>都</strong>是 16 位。<ol><li>8 位：AL 中和 8 位寄存器或内存<strong>字节</strong>单元中；</li><li>16 位：AX 中和 16 位寄存器或内存<strong>字</strong>单元中。</li></ol></li><li>结果<ol><li>8 位：AX 中；</li><li>16 位：DX（高位）和 AX（低位）中。</li></ol></li></ol><p>  格式如下</p><pre><code class=language-x86asm>mul reg
mul 内存单元
</code></pre><p>  内存单元可以用不同的寻址方式给出，比如</p><pre><code class=language-x86asm>mul byte ptr ds:[0]     # 含义为：(ax)=(al)×((ds)×16+0)
mul word ptr [bx+si+8]
含义为：
(ax)=(ax)×((ds)×16+(bx)+(si)+8) 结果的低 16 位；
(dx)=(ax)×((ds)×16+(bx)+(si)+8) 结果的高 16 位；
</code></pre><p>  例如</p><ol><li><p>计算 100×10
100 和 10 小于 255，可以做 8 位乘法，程序如下</p><pre><code class=language-x86asm>mov al,100
mov bl,10
mul bl
结果：(ax)=1000（03E8H）
</code></pre></li><li><p>计算 100×10000
100 小于 255，可 10000 大于 255，所以必须做 16 位乘法，程序如下</p><pre><code class=language-x86asm>mov ax,100
mov bx,10000
mul bx
结果：(ax)=4240H，(dx)=000FH（F4240H=1000000）
</code></pre></li></ol><h3 id=109-模块化程序设计>10.9 模块化程序设计</h3><p>  从上面我们看到，call 与 ret 指令共同支持了汇编语言编程中的模块化设计。在实际编程中，程序的模块化是必不可少的。</p><p>  因为现实的问题比较复杂，对现实问题进行分析时，把它转化成相互联系、不同层次的子问题，是必须的解决方法。</p><p>  而 call 和 ret 指令对这种分析方法提供了程序实现上的支持。利用 call 和 ret 指令，我们可以用简洁的方法，实现多个互相联系、功能独立的子程序来解决一个夫复杂的问题。</p><h3 id=1010-参数和结果传递的问题>10.10 参数和结果传递的问题</h3><p>  子程序一般都要根据提供的参数处理一定的事务，处理后，将结果（返回值）提供给调用者。</p><p>  其实，我们讨论参数和返回值传递的问题，实际上就是在探讨，应该如何存储子程序需要的参数和产生的返回值。</p><p>  我们设计一个子程序，可以根据提供的 N，来计算 N 的 3 次方。</p><p>  这里有两个问题</p><ol><li>我们将参数 N 存储在什么地方？</li><li>计算得到的数值，我们存储在什么地方？</li></ol><p>  很显然，我们可以用寄存器来存储，可以将参数放到 bx 中；</p><p>  因为子程序中要计算 N×N×N，可以使用多个 mul 指令，为了方便，可将结果放到 dx 和 ax 中。</p><p>  子程序</p><ol><li><p>说明：计算 N 的 3 次方</p></li><li><p>参数：(bx)=N</p></li><li><p>结果：(dx:ax)= $ N^3 $</p><pre><code class=language-x86asm>cube:mov ax,bx
	mul bx
	mul bx
	ret
</code></pre></li></ol><blockquote><p>我们在编程的时候要注意良好的风格，对于程序应有详细的注释。子程序的注释信息应该包含对子程序的功能、参数和结果的说明。</p></blockquote><p>  用寄存器来存储参数和结果是最常使用的方法。对于存放参数的寄存器和存放结果的寄存器，调用者和子程序的读写操作恰恰相反</p><ol><li>调用者将参数送入参数寄存器，从结果寄存器中取到返回值；</li><li>子程序从参数寄存器中取到参数，将返回值送入结果寄存器。</li></ol><h3 id=1011-批量数据的传递>10.11 批量数据的传递</h3><p>  前面的例程中，子程序 cube 只有一个参数，放在 bx 中。如果有两个参数，那么可以用两个寄存器来放，可是如果需要传递的参数有 3 个、4 个或更多直至 N 个，我们怎样存放呢？</p><p>  寄存器的数量终究有限，我们不可能简单地用寄存器来存放多个需要传递的数据。对于返回值，也有同样的问题。</p><p>  在这种时候，我们将批量数据放到内存中，然后将它们所在内存空间的首地址放在寄存器中，传递给需要的子程序。</p><p>  对于具有批量数据的返回结果，也可用同样的方法。</p><p>  编程：将 data 段中的字符串转化为大写。</p><pre><code class=language-x86asm>assume cs:code

data segment
	db 'conversation'
date ends

code segment

start:
	mov ax,data
	mov ds,ax
	mov si,0     # ds:si 指向字符串（批量数据）所在空间的首地址

	mov cx,12     # cx 存放字符串的长度
	call capital

	mov ax,4c00h
	int 21h

capital:
	and byte ptr [si],11011111b
	inc si
	loop capital
	ret

code ends

end start
</code></pre><blockquote><p>除了寄存器、内存传递参数外，还有一种通用的方法使用栈来传递参数。关于这种技巧请参看附注 4。</p></blockquote><h3 id=1012-寄存器冲突的问题>10.12 寄存器冲突的问题</h3><p>  设计一个子程序</p><ol><li><p>功能：将一个全是字母，以 0 结尾的字符串，转化为大写。</p></li><li><p>程序要处理的字符串以 0 作为结尾符，这个字符串可以如下定义</p><pre><code class=language-x86asm>db 'conversation',0
</code></pre></li></ol><p>  分析</p><ol><li>应用这个子程序，字符串的内容后面定要有一个 0，标记字符串的结束。子程序可以依次读取每个字符进行检测，如果不是 0，就进行大写的转化，如果是 0，就结束处理。</li><li>由于可通过检测 0 而知道是否已经处理完整个字符串，所以子程序可以不需要字符串的长度作为参数。我们可以直接用 jcxz 来检测 0。</li></ol><p>  子程序实现代码</p><pre><code class=language-x86asm>capital:
	mov cl,[si]
	mov ch,0
	jcxz ok     # 如果 (cx)=0，结束；如果不是 0，处理。
	and byte ptr [si],11011111b     # 将 ds:si 所指单元中的字母转化为大写
	inc si     # ds:si 指向下一个单元
	jmp short capital
ok:ret
</code></pre><p>  子程序的应用</p><p>  将 data 段中的字符串全部转化为大写</p><pre><code class=language-x86asm>assume cs:code
data segment
	db 'word',0
	db 'unix',0
	db 'wind',0
	db 'good'.0
data ends
</code></pre></article><div class=my-4><a href=https://b.hui.ke/tags/mul/ class="inline-block bg-tertiary-bg text-sm rounded px-3 py-1 my-1 me-2 hover:text-eureka">#mul</a>
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<a href=https://b.hui.ke/docs/assembly-language/16/ class=block>11.1 ~ 11.6</a></div><div class="mt-4 md:mt-0 md:text-right"><span class="text-primary-text block font-bold">Next</span>
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